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Are surface-scanning satellites able to show certain parts of Earth's surface real-time? For example, to follow a person who's escaping.

Somewhere I heard that these satellites are able to make pictures only, but unable to make constant video stream. In fact, 20-30 pictures per seconds might be enough to create the feel of moving.

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    $\begingroup$ There is no difference between a "constant video stream" and 20-30 images per second. That's what a video is. $\endgroup$ – Vortico Apr 29 '14 at 6:16
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Satellites are nowadays definitely not bandwidth starved, considering we've just had a few launches in the past month orbiting satellites capable of relaying live telecasts even in quad HD resolution. But the problem with tracking someone on the ground in real-time is more one of radial resolution, than the communications capabilities of satellites.

You see, the Geostationary Orbit (GEO) is at an altitude of roughly 35,786 kilometres (22,236 mi) above the Earth's equator, so that's quite a long distance and you'd need a telescope in the range of the Hubble Space Telescope to see much detail on the ground. Obviously, we're talking of a huge mass satellite then, which doesn't make the task of orbiting at least 3 of them any easier.

                                                            GEO

So alternatively, you could use a cluster of satellites that orbit at a lot lower altitudes in Low Earth Orbit, say around 500 km above the Earth's surface. Their radial resolution could be a lot better even by using fairly small (in relative terms) lenses, but they will pass one specific point on the surface of the Earth a lot faster, so you'd want to move the task of tracking a single point from one satellite to another to keep your target in their line of sight. They would also have to move the lense constantly, to adjust for the fast changing angle of a relatively stationary target on the surface of the Earth.

   LEO

So the answer to your question is not quite as straightforward as you might have hoped it would be. It is possible, but would require either a huge telescope-like tracking satellite in the GEO (and even then the object you're tracking has to be observable from the position the satellite is), or you will have to delegate this task of tracking an object on the surface of the Earth in real-time to several satellites that follow each other's orbit at regular intervals, enabling covering of a single point on the Earth at all times.

But regardless if you're using a few huge GEO satellites, or a large number of smaller LEO ones, they still can't make clouds move away from being over your target you're tracking. You could of course use different electromagnetic wavelengths than those in visible light spectrum, and hope they penetrate lower atmosphere weather formations easier, but it wouldn't really look like the orthophoto images on Google Maps, for example.

For the most part, it's just easier, faster and cheaper to deploy reconnaissance airplanes over your target, or if you're on a tight budget, maybe paint a blimp with some advert nobody cares to give attention to, and have your cameras hover a lot closer to the target you'd like to track and avod the cloud cover. But it is certainly not impossible to reconfigure a large array of satellites on the fly, although you better hope your boss didn't want you to read your target's newspaper through the clouds, or at an angle with direct line of sight.

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    $\begingroup$ You could of course use different electromagnetic wavelengths than those in visible light spectrum, and hope they penetrate lower atmosphere weather formations easier; at any wavelength long enough to penetrate clouds (i.e. microwaves), you can't remotely get a high enough resolution to detect people, due to the diffraction limit. $\endgroup$ – gerrit Sep 2 '13 at 13:27
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    $\begingroup$ @gerrit - I've already mentioned that problem, granted, in a bit more liberal form: "...although you better hope your boss didn't want you to read your target's newspaper through the clouds..." ;) $\endgroup$ – TildalWave Sep 2 '13 at 14:16
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    $\begingroup$ Nice use of xkcd images:-) $\endgroup$ – Rory Alsop Sep 2 '13 at 15:30
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Short Answer

Yes.

Theory Answer

Satellite Television is broadcast at full 1080P high definition (1980x1080 px + sound + Closed Captioning + Program Metadata) data rates with many channels per satellite — 50+ — and is also receiving at a slightly higher rate (Data In = Data Out + control data on a separate channel).

Likewise, HDTV quality signal is well within the bounds of fielded space-rated CCD video cameras.

Both can be operated on reasonable solar panel loads.

Practical Factor Answer

But, theory aside, It has been done in the past.

The Space Shuttle has often broadcast live NTSC feeds to earth during it's operational tenure. Most, but not all, have been crew interviews. Some have been observations using outboard cameras, including the live feed of the Hubble Repair.

By simply using a different lens set, namely changing to a narrow field telephoto lens, a mounted camera could be pointed at specific features on planet and used to provide live surveillance.

And History leading back to more Theory

Note that such cameras are a "open secret" - Military satelite video has been released showing roughly 0.5m per pixel NTSC video (29.97FPS with roughly 720x480) since the early 1990's. Given the slightly better than 0.1m resolution of civilian mapping photography (EG: Google Earth), it shows that the cameras can physically resolve that, so it's widely supposed that 0.1m per pixel video feed is within the capabilities of military spy satellites. And given that the broadcast rate is capable of far more, it's unlikely that the military does not have at least that level of capability.

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Your question has 2 main points:

  1. Can satellites record video?

Both Pleaides and Skysat have the capability to record videos.

  1. Can satellites downlink real-time payload data at the required throughput for video?

AFAIK, Skybox requires pointing to its ground station, while I suspect Pleiades does not need to do so.

Theoretically then (and up to SW updates and ground infrastructure), Pleiades could send real-time video from orbit, tracking a target. That would require having target visibility and ground station visibility at the same time (would last just around 10min).

Furthermore...

The technology is in place to create what you desire, as some satellites have inter-satellite laser communication such as SPOT-5. This allows a satellite to downlink it's data not to a ground station directly, but rather to a geostationary satellite.

However...

There is still the limitation of the target's visibility from the satellite (still around 10min). This one could be solved by having lots of satellites dedicated to this mission, again theoretically possible, but financially implausible, as ideas for large constellations of big satellites usually end up badly.

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For short videos of Earth this can be done with surprisingly nice results from low Earth orbit (LEO) with a little image processing to flatten and align the image and generate the appearance that the spacecraft is hovering.

These videos are only about 15 or 20 seconds and you can't choose a place and time arbitrarily, so I don't think this technique offers what the OP is looking for currently.


From the question Hovering Carbonite! Why do these satellite videos of Earth appear to be made from a geostationary location?:

The BBC News article UK satellite makes HD colour movies of Earth has led me to a series of YouTube videos as well recorded by the Carbonite-2 spacecraft built by Surrey Satellite Technology to be operated by Earth-i.

The first video of Buenos Aires is quite striking, it looks like the viewpoint is not moving, even though the spacecraft is in a nearly circular LEO at about 500 km. https://www.n2yo.com/satellite/?s=43115

Related: Can image-stacking allow this 0.25 m satellite telescope achieve 0.65m resolution?

Open the videos in a new tab/window and set to 1080p resolution and full screens if you've got 'em.


From this answer

@BowlOfRed's comment nailed it.

I'd thought about making an optical flow version but time does not allow, so I've just used screenshots and imageio found here.

It looks like the spacecraft was overhead, and in fact did move many degrees (as seen from the ground) during the 17 second video of Buenos Aires.

enter image description here

enter image description here

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